The present invention generally relates to an electromagnetic tracking system. In particular, the present invention relates to a system and method for the use of multiple coil architectures simultaneously in one electromagnetic (“EM”) tracking system.
Medical practitioners, such as doctors, surgeons, and other medical professionals, often rely upon technology when performing a medical procedure, such as image-guided surgery (“IGS”) or examination. An IGS system can provide positioning and/or orientation (“P&O”) information for the medical instrument with respect to the patient or a reference coordinate system, for example. A medical practitioner can refer to the IGS system to ascertain the P&O of the medical instrument when the instrument is not within the practitioner's line of sight with regard to the patient's anatomy, or with respect to non-visual information relative to the patient. An IGS system can also aid in pre-surgical planning.
The IGS or navigation system allows the medical practitioner to visualize the patient's anatomy and track the P&O of the instrument. The medical practitioner can use the tracking system to determine when the instrument is positioned in a desired location or oriented in a particular direction. The medical practitioner can locate and operate on, or provide therapy to, a desired or injured area while avoiding other structures. Increased precision in locating medical instruments within a patient can provide for a less invasive medical procedure by facilitating improved control over smaller, flexible instruments having less impact on the patient. Improved control and precision with smaller, more refined instruments can also reduce risks associated with more invasive procedures such as open surgery.
In medical and surgical imaging, such as intraoperative or perioperative imaging, images are formed of a region of a patient's body. The images are used to aid in an ongoing procedure with a surgical tool or instrument applied to the patient and tracked in relation to a reference coordinate system formed from the images. Image-guided surgery is of a special utility in surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac procedures, interventional radiology and biopsies in which x-ray images can be taken to display, correct the P&O of, or otherwise navigate a tool or instrument involved in the procedure.
Generally, image-guided surgery systems operate with an image display which is positioned in a surgeon's field of view and which displays a few panels such as a selected MRI image and several x-ray or fluoroscopic views taken from different angles. In tool navigation systems, the display visible to the surgeon may show an image of a surgical tool, biopsy instrument, pedicle screw, probe or other device projected onto a fluoroscopic image, so that the surgeon can visualize the orientation of the surgical instrument in relation to the imaged patient anatomy. An appropriate reconstructed CT or MRI image, which may correspond to the tracked coordinates of the probe tip, can also be displayed.
Among the systems that have been proposed for effecting such displays, many rely on closely tracking the position and orientation of the surgical instrument in external coordinates. The various sets of coordinates can be defined by robotic mechanical links and encoders, or more usually, are defined by a fixed patient support, two or more receivers such as video cameras which can be fixed to the support, and a plurality of signaling elements attached to a guide or frame on the surgical instrument that enable the position and orientation of the tool with respect to the patient support and camera frame to be automatically determined by triangulation, so that various transformations between respective coordinates can be computed.
The highly accurate tracking technology found in navigation systems can also be used to track the P&O of items other than medical instruments in a variety of applications. That is, a tracking system can be used in other settings where the P&O of an object in an environment is difficult to accurately determine by direct or indirect inspection. For example, tracking technology can be used in forensic or security applications. Retail stores can use tracking technology to prevent theft of merchandise. In such cases, a passive transponder can be located on the merchandise. A transmitter can be strategically located within the retail facility. The transmitter emits an excitation signal at a frequency that is designed to produce a response from a transponder. When merchandise carrying a transponder is located within the transmission range of the transmitter, the transponder produces a response signal that is detected by a receiver. The receiver then determines the location of the transponder based upon characteristics of the response signal.
Tracking systems are also often used in virtual reality systems or simulators. Tracking systems can be used to monitor the position of a person in a simulated environment. A transponder or transponders can be located on a person or object. A transmitter emits an excitation signal and a transponder produces a response signal. A receiver detects the response signal. The signal emitted by the transponder can then be used to monitor the position of a person or object in a simulated environment.
Tracking systems can be optical, ultrasonic, inertial, or electromagnetic, for example. Electromagnetic tracking systems can employ coils as receivers and transmitters. In EM trackers, transmitter coil or coils emit quasi-static magnetic fields. In addition, receiver coil(s) measure these fields. From the field measurements and mathematical models of the coils, the position and orientation of the receiver with respect to the transmitter can be determined. Alternatively, the position and orientation of the transmitter with respect to the receiver is determined.
Typically, an electromagnetic tracking system is configured in an industry-standard coil architecture (“ISCA”). ISCA trackers use a trio of nearly-colocated, nearly-orthogonal, nearly-dipole coils for the transmitter and another trio of nearly-colocated, nearly-orthogonal, nearly-dipole coils for the receiver. Each coil trio is carefully characterized during manufacture to numerically express the precise value of the “nearly” in the previous sentence. From the field measurements and mathematical models of the coils, the position and orientation of the receiver with respect to the transmitter is determined. Alternatively, the position and orientation of the transmitter with respect to the receiver is determined. All six degrees of freedom (three of position and three of orientation) are tracked.
Single-coil EM trackers use a single dipole or nearly-dipole transmitter coil and an array of six or more receiver coils, or else use a single dipole or nearly-dipole receiver coil and an array of six or more transmitter coils. By electromagnetic reciprocity, these two arrangements function equivalently. The coils in the array can be dipole, nearly-dipole, or non-dipole coils (or combinations). The coils in the array are either precisely manufactured or precisely characterized during manufacture to obtain mathematical models of the coils in the array. The single coil does not need to be characterized. From the field measurements and mathematical models, the position and orientation of the single coil with respect to the array are tracked. Since the single coil is symmetrical about its roll axis, only five degrees of freedom (six of position and two of orientation) of position and orientation are tracked. The gain of the single coil can also be tracked.
The array of coils can be fabricated as a printed-circuit board or as an array of wound coils or as a combination of both. Arrangements of coils in the array vary widely in various implementations of single-coil EM trackers. The array can include electrically-conductive or ferromagnetic materials as part of the design of the array.
The big disadvantage of single-coil EM trackers is the need to find a place to put the array of coils. To work well, this array needs to be physically spread out in space.
Note that a multichannel single-coil EM tracker can track two or three single coils simultaneously. If two or more single coils are mounted rigidly with respect to each other with their axes pointed in different directions, and tracked as two or more single coils or as a group, all six degrees of freedom could be tracked for the set of single coils.
In current systems and methods employing a fluoroscope, an ISCA transmitter is rigidly mounted on the relevant anatomy of a patient to serve as a dynamic reference. One or two ISCA receivers are mounted on the x-ray image detector of a fluoroscope. The ISCA receivers are spaced away from the surface of the detector, to reduce the effects of field distortion.
When taking a fluoroscopic image, the detector can be tracked with respect to the ISCA transmitter in order to determine the position and orientation of the image intensifier with respect to the relevant anatomy of the patient.
One or two ISCA receivers are also mounted on a surgical instrument being navigated. The instrument ISCA receivers can then be tracked with respect to the ISCA transmitter in order to determine the position and orientation of the surgical instrument with respect to the relevant anatomy of the patient. The real-time position and orientation of the surgical instrument is then calculated and displayed on stored fluoroscopic images.
One major difficulty with current systems and methods is that the electrically-conductive materials and ferromagnetic materials in the fluoroscope detector distort the magnetic fields near the ISCA receivers mounted on the detector. ISCA EM trackers are very sensitive to such field distortion, leading to inaccurate tracking or failure to track at all in severe cases. However, in general, the detector can be far enough from the instrument that the detector does not significantly distort the fields at the ISCA receiver(s) mounted on the surgical instrument.
Current solutions to this difficult include the use of manufacturing-time-intensive robotic mapping procedures to correct the tracking errors. However, such systems and methods must still be used so as to avoid positions and orientations of instruments, patient anatomies, and/or detectors that result in failure to track. Therefore, users of such systems and methods end up with a situation where (1) the detector cannot be tracked in some desired positions and orientations, (2) the receivers stick out from the detector so as to avoid interference and, in doing so, get in the way of medical or imaging procedures, and/or (3) an expensive mapping manufacturing process is necessary.
Another current area of application of current systems and methods is the use of a surgical microscope for surgery inside a skull. Here, the fluoroscope described above is typically replaced with a microscope.
In this application, an ISCA transmitter is rigidly fixed to the skull to provide the dynamic reference to the patient's anatomy. One or two ISCA receivers are attached to the surgical instrument to track the position and orientation of the instrument with respect to the ISCA transmitter, and thus with respect to the patient's anatomy. The real-time position and orientation of the instrument are superimposed on pre-operative images of the patient's anatomy, similar to as described above.
One or two ISCA receivers are mounted on the surgical microscope to permit tracking the microscope's line-of-sight with respect to the ISCA transmitter, and thus with respect to the patient's anatomy.
The position of the microscope's focal point along the microscope's line-of-sight can be read from the microscope. This information permits the position of the microscope's focal point to be determined with respect to the ISCA transmitter, and thus with respect to the patient's anatomy.
The real-time position and orientation of the microscope's focal point and focal axis can then be superimposed on pre-operative images of the patient's anatomy. The real-time position and orientation of the instrument can also be superimposed on pre-operative images of the patient's anatomy.
In general, the microscope contains a lot of electrically-conductive material which distorts the magnetic fields near the microscope, therefore leading to tracking errors. To avoid inaccurate tracking, the ISCA receivers must be then mounted spaced away from the microscope. The “spaced-away” receivers can get in the way of the surgeon's work. On the other hand, the microscope is far enough away from the ISCA transmitter and from the ISCA receivers mounted on the surgical instrument so that the microscope does not significantly distort the magnetic fields measured by the ISCA receivers mounted on the surgical instrument.
Thus, a need exists for a navigation system and method employing a tracking technology that reduces the amount of interference experienced by trackers so as to increase the accuracy and precision of such a system and method and to decrease the amount of space required for such a system and method. Single-coil EM trackers are significantly less susceptible to the field-distorting effects of nearby materials than are ISCA EM trackers. Therefore, by converting systems and methods such as fluoroscope and microscope applications from ISCA EM tracking to single-coil EM tracking can reduce the space required and the amount of interference.
However, in order to perform such a conversion, one would need to first convert the ISCA transmitter to an ISCA receiver, then mount a new array of transmitter coils close to the working volume but out of the way of the surgeon. In addition, one would need to add tracker electronics to drive the new array of transmitters, add tracker electronics to receive the signals from the new receiver, and modify the tracker software to track each ISCA receiver as a group of single coils and to calculate the position and orientation (all six degrees of freedom) of each ISCA receiver. Yet, the big disadvantage of such a conversion is the need to find a place to put the array of transmitter coils. To work well, this array needs to be physically spread out in space.
Another solution could be to employ a system and method that permits tracking of ISCA trackers and single-coil trackers one at a time. In such a system and method, a user of the system would need to continually switch back and forth between tracking the position and orientation of the ISCA trackers and the position and orientation of the single-coil trackers. This back and forth switching would not permit continuous tracking of both the ISCA trackers and single-coil trackers and would contribute additional complexity to the system and method. Therefore, a need exists for a system and method that can simultaneously track multiple coil architectures.